It is often desirable to heat-treat metal strip in coil form under controlled atmosphere conditions. Historically this was accomplished with steel strip by placing a coil on end on a stool, or a stack of coils on end, one on top of the other, by closing them with an open-bottom cover, usually called a bell, and heating the stool, coil or coils and bell in a furnace. The bell was sealed to the stool during that heating and the desired atmosphere maintained by introducing an appropriate gas into the bell under a pressure above that obtaining in the furnace. The coils were spaced from each other and the stool by separators in grating form so as to allow circulation of the atmosphere in the bell. Aluminum, however, is much softer than steel. Aluminum coils cannot be heated at temperatures approaching full anneal (about 735 degrees F.) standing on end or on gratings without incurring severe damage to the wraps. Historically, therefore, aluminum in coils has been heat-treated by placing each coil on its side on a car, pulling a string of such loaded cars into a long furnace and raising or lowering the furnace temperature to obtain the desired heat-treating cycle.
Both processes above-mentioned require that the furnace and its entire burden be raised to the desired heat-treating temperature, held at that temperature for the required length of time and cooled to the desired discharge temperature for each furnace charge. The cycle is then repeated for the next furnace charge. It is evident that such processes cannot be economical of heat.